Despite several days worth of testing, fiddling and communicating on Yahoo groups I have not managed to solve my problem with MaxIm. When combining individual flat field frames with sigma clipping or SD masking the resulting flat field calibration fails to remove dust doughnuts. Working with the same data in Mira I have no problems. I will now generate my dark corrected master flat field in Mira, then use it in MaxIm. In future sessions I will try to use Maxim again - if it continues to fail I'll have to contact Cyanogen directly. Irritating!!!